1. Field of the Invention
The present invention relates to an adsorbent for nitrogen oxides discharged from automobiles and heat generators, and a method of treating it to improve its adsorptivity.
2. Discussion of Background
Environmental pollution problems are not problems restricted to a narrow zone now, and have developed into big problems to be dealt with globally, and one of which is air pollution. In exhaust gas generated by artificially burning fuel, substances artificially synthesized and decomposition products thereof, substances hazardous to the human body are contained. It is hard to completely remove such substances from sources of release, and they are discharged to living environment in fact. The amount of such hazardous gas discharged increases along with activation of human activity. For example, generation of nitrogen oxide as one of air pollutants is due to exhaust gas from automobiles and heat generators. Along with growth in traffic, area in which the nitrogen oxide concentration exceeds environmental standard as established by Environment Agency of Japan is increasing in urban area.
Under these circumstances, it is necessary to improve technique to remove hazardous gas from sources of release for a comfortable life. Adsorbents for nitrogen oxides reported up to now are ones having a noble metal or its oxide immersed in or supported by a carrier. Among them, three way catalysts used currently can not be used for treatment of exhaust gas containing a large amount of oxygen, such as exhaust gas of e.g. a lean burn engine, since their nitrogen oxide selective reduction activity substantially disappears by presence of only 0.5% of excess oxygen in the exhaust gas, with regard to both Rh type and non-Rh type. Further, zeolite subjected to ion exchange has a low heat resistance, its crystal structure tends to disintegrate when used at a high temperature of at least 700xc2x0 C., and there are problems in NO adsorption and decomposition activity.
As a material expected as an adsorbent for nitrogen oxides which is a carrier-free adsorbent, amorphous calcium phosphate particles are mentioned (JP-A-7-55586). However, if the heat treatment temperature exceeds 800xc2x0 C., the amorphous calcium phosphate particles undergo crystallization to decrease their adsorptivity of a gas.
It has been reported that La2O3 adsorbs nitrogen monoxide recently (B. Klingenberg, et al., Appl. Catal. B: Environmental 21, 19 (1999)), but no function regarding adsorption when oxygen is contained, has been reported.
The present invention has been made to overcome the above problems, and it is an object of the present invention to provide an excellent adsorbent for nitrogen oxides and a pretreatment method of the adsorbent, free from the above problems.
The present inventors have conducted extensive studies to achieve the above object and as a result, have found that functions of the following material as an adsorbent for nitrogen oxides can be improved by employing the following conditions. The present invention has been accomplished on the basis of this discovery.